1. Field of the Invention
The present invention relates to ultrasonic sensors, and more particularly, to an ultrasonic sensor preferably for use as, for example, a backup sensor of an automobile.
2. Description of the Related Art
FIG. 6 is a schematic illustration showing an exemplary ultrasonic sensor according to a related art. An ultrasonic sensor 1 includes a cylindrical case 2 with a bottom portion, the case 2 being made of aluminum or other suitable material. An inner side of a bottom of the case 2 is connected to one side of a piezoelectric element 3. The case 2 is substantially entirely filled with foamable resin 4, such as foamable silicon, so as to surround the piezoelectric element 3. Also, a substrate 6 having terminals 5a and 5b is attached to an opening of the case 2 so as to cover the foamable resin 4. Provided on both sides of the substrate 6 are electrodes 7a and 7b which are connected to the terminals 5a and 5b. The one terminal 5a is connected to the other side of the piezoelectric element 3 via the electrode 7a provided on the inner side of the substrate 6, and via a wire 8. The other terminal 5b is connected to the one side of the piezoelectric element 3 via the electrode 7b provided on the outer side of the substrate 6, via a solder 9, and via the case 2.
Using the ultrasonic sensor 1 to measure the distance between the ultrasonic sensor 1 and an object to be detected, the piezoelectric element 3 is excited by applying a drive voltage to the terminals 5a and 5b. As the piezoelectric element 3 is vibrated, the bottom of the case 2 is vibrated, which causes ultrasound to be emitted to a direction substantially perpendicular to the bottom as indicated by an arrow shown in FIG. 6. When the ultrasound emitted from the ultrasonic sensor 1 is reflected by the object to be detected and reaches the ultrasonic sensor 1, the piezoelectric element 3 is vibrated, the vibration is converted into an electric signal, and the electric signal is output from the terminals 5a and 5b. A period of time from the application of the drive voltage to the output of the electric signal is measured, so that a distance between the ultrasonic sensor 1 and the object to be detected may be measured.
With the ultrasonic sensor 1, since the inside of the case 2 is filled with the foamable resin 4, the vibration of the entire case 2 may be reduced. Also, the ultrasound emitted to the inside of the case 2 is dispersed and absorbed by a large number of pores in the foamable resin 4. This may efficiently reduce not only the vibration of the case 2, but also the ultrasound remaining in the case 2, thereby improving the reverberation property (see Japanese Unexamined Patent Application Publication No. 11-266498).
To fill the case with the foamable resin, if the resin is injected into the case and then foamed before the substrate is attached to the case, then the foamable resin may come out from the opening of the case. As a result, only a small internal pressure is applied. Accordingly, the case may not be evenly filled with the foamable resin at the corners thereof. If the case is not evenly filled with the foamable resin, the reverberation property may be deteriorated. Also, if the resin is injected into the case and is foamed after the substrate is attached to the case such that the internal pressure is increased, the foamable resin pushes the substrate from the inside of the case, the substrate is deformed, and thus, the foamable resin may be unevenly filled in the case.